Microchannel plates are used as electron multipliers in image intensifiers. They are thin glass plates having an array of channels extending there through, which are located between a photocathode and a phosphor screen. An incoming electron from the photocathode enters the input side of the microchannel plate and strikes a channel wall. When voltage is applied across the microchannel plate, these incoming or primary electrons are amplified, generating secondary electrons. The secondary electrons then exit the channel at the back end of the microchannel plate and generate an image on the phosphor screen.
In general, fabrication of a microchannel plate starts with a fiber drawing process, as disclosed in U.S. Pat. No. 4,912,314, issued Mar. 27, 1990 to Ronald Sink, which is incorporated herein by reference in its entirety. For convenience, FIGS. 1-4, disclosed in U.S. Pat. No. 4,912,314, are included herein and discussed below.
FIG. 1 shows a starting fiber 10 for the microchannel plate. Fiber 10 includes glass core 12 and glass cladding 14 surrounding the core. Core 12 is made of glass material that is etchable in an appropriate etching solution. Glass cladding 14 is made from glass material which has a softening temperature substantially the same as the glass core. The glass material of cladding 14 is different from that of core 12, however, in that it has a higher lead content, which renders the cladding non-etchable under the same conditions used for etching the core material. Thus, cladding 14 remains after the etching of the glass core. A suitable cladding glass is a lead-type glass, such as Corning Glass 8161.
The optical fibers are formed in the following manner: An etchable glass rod and a cladding tube coaxially surrounding the rod are suspended vertically in a draw machine which incorporates a zone furnace. The temperature of the furnace is elevated to the softening temperature of the glass. The rod and tube fuse together and then are drawn into a single fiber 10. Fiber 10 is fed into a traction mechanism, in which the speed is adjusted until the desired fiber diameter is achieved. Fiber 10 is then cut into shorter lengths of approximately 18 inches.
Several thousands of the cut lengths of single fiber 10 are then stacked into a graphite mold and heated at a softening temperature of the glass to form hexagonal array 16, as shown in FIG. 2. As shown, each of the cut lengths of fiber 10 has a hexagonal configuration. The hexagonal configuration provides a better stacking arrangement.
The hexagonal array, which is also known as a multi assembly or a bundle, includes several thousand single fibers 10, each having core 12 and cladding 14. Bundle 16 is suspended vertically in a draw machine and drawn to again decrease the fiber diameter, while still maintaining the hexagonal configuration of the individual fibers. Bundle 16 is then cut into shorter lengths of approximately 6 inches.
Several hundred of the cut bundles 16 are packed into a precision inner diameter bore glass tube 22, as shown in FIG. 3. The glass tube is made of a glass material similar to glass cladding 14 but is non-etchable by the etching process used to etch glass core 12. The outer glass tube 22 eventually becomes a solid rim border of the microchannel plate.
In order to protect fibers 10 of each bundle 16, during processing to form the microchannel plate, several support structures are positioned in glass tube 22 to replace those bundles 16 which form the outer layer of the assembly. The support structures may take the form of hexagonal rods of any material having the necessary strength and the capability to fuse with the glass fibers. Each support structure may be a single optical glass fiber 24 having a hexagonal shape and a cross-sectional area approximately as large as that of one of the bundles 16. The single optical glass fiber, however, has a core and a cladding which are both non-etchable. The optical fibers 24, or support rods 24, are illustrated in FIG. 3, as disposed at the periphery of assembly 30 surrounding the many bundles 16.
The support rods may be formed from one optical fiber or any number of fibers up to several hundred. The final geometric configuration and outside diameter of one support rod 24 is substantially the same as one bundle 16. The multiple fiber support rods may be formed in a manner similar to that of forming bundle 16.
Each bundle 16 that forms the outermost layer of fibers in tube 22 is replaced by a support rod 24. This is preferably done by positioning one end of a support rod 24 against one end of a bundle 16 and then pushing support rod 24 against bundle 16, until bundle 16 is out of tube 22. The assembly formed when all of the outer bundles 16 have been replaced by support rods 24 is called a boule, and is generally designated as 30 in FIG. 3.
Boule 30 is fused together in a heating process to produce a solid boule of rim glass and fiber optics. The fused boule is then sliced, or diced, into thin cross-sectional plates or wafers. The wafers are ground and polished.
In order to form the microchannels, cores 12 of optical fibers 10 are removed, by etching with dilute hydrochloric acid. After etching the boule, the high lead content glass cladding 14 remains to form microchannels 32, as illustrated in FIG. 4. Also, support rods 24 remain solid and provide a good transition from the solid rim of tube 22 to microchannels 32.
Additional process steps include beveling and polishing of the glass boule. After the plates are etched to remove the core rods, the channels in the boule are metalized and activated.
In the fabrication of a microchannel plate, the core/clad rods are typically stacked into a symmetric hexagonal shape, as described above with respect to FIG. 2 and shown as a top view in FIG. 5A. In the interior of bundle 16, each core/clad rod 10 is represented by a circle, designated as 10. The circles are tightly packed into a hexagonal shape.
If each circle in FIG. 5A represents a core/clad pair, then broken channel walls may occur when the clad walls etch out and the circles touch each other. The maximum possible open area ratio (OAR), just before this break through, may be calculated, using the geometry shown in FIG. 5B, where r is the radius of circle 10, as follows:
            Channel      ⁢                          ⁢      area        =          Π      ⁢                          ⁢                        r          2                /        2                        Dashed      ⁢                          ⁢      area        =          2      ⁢              (                              1            2                    ⁢                      3                    ⁢                      r            2                          )                                                      Area            ⁢                                                  ⁢            ratio                    =                    ⁢                      Π            ⁢                                                  ⁢                                          r                2                            /              2                        ⁢                          3                        ⁢                          r              2                                                                    =                    ⁢                      Π            /            3.46                                          Maximum      ⁢                          ⁢      OAR        =          90.69      ⁢      %      
As bundles 16 are stacked to form a boule, for example boule 30, multiple hexagonal shaped bundles 16 (FIG. 5A) are stacked and pressed together to form multiboundary regions, as shown in FIG. 6A. These multiboundary regions are designated as 60.
Upon careful examination of FIG. 6A, it may be observed that multiboundary regions 60 are easily differentiated from the interior of each hexagonal multifiber, or bundle 16. At the interface between each bundle, fibers 10 are packed in a square pack arrangement. The maximum OAR for the square pack arrangement may be calculated using the geometric relationship shown in FIG. 6B, where r is the radius of circle 10, as follows:Channel area=Πr2 Dashed area=(2r)2 Area ratio=Πr2/(4r2)=Π/4Maximum OAR=78.5%
It will be appreciated that there is a large difference in the maximum OAR achievable between the packing of rows in hexagonal array 16 (FIG. 5A) and the packing of a square array of rows at multiboundary regions 60 (FIG. 6A). The former achieves a maximum OAR of 90.7% and the latter achieves only a maximum OAR of 78.5%.
Since there must be a safety margin of achievable OAR (material is needed at the interface between each fiber), current boules are formed to achieve 63% OAR. In addition, there may be the occurrence of broken channel walls.
The present invention, as will be described, provides a method of stacking the bundles, so that the square packs of rows at the multiboundary regions of the boule are minimized or eliminated. This, in turn, increases the OAR during the boule fabrication. The present invention provides a boule which continues the hexagonal close packing of rows across the multiboundary regions. In addition, advantageously, the bundles need not be shifted by half a channel, one bundle to an adjacent bundle. The present invention is described below.